This invention relates to a method of making a magnetic recording medium and more particularly relates to a method suitable for continuously depositing a ferromagnetic layer on a flexible substrate such as a plastic film.
Vacuum deposition, sputtering, ion plating, etc. have been known as a method of making a ferromagnetic thin film layer mainly composed of iron group metals. The method most practical for an industrial-scale manufacturing is vacuum deposition process employing an eletron beam for heating an evaporation source material. However the electron beam heating has not been fully developed yet because of its short history, and especially there scarcely exists an achievement in a continuous deposition.
When an evaporation source material is heated by electron beam, a temperature thereof becomes fairly high. It possibly causes a container for the source material to be melted, whereby it is destroyed in a short time or component materials thereof is included into deposited ferromagnetic layer, so as to deteriorate magnetic properties of the layer. Therefore what material the container is made of has been an important problem.
A water-cooled copper hearth has hitherto been used as the container having a heat resistance sufficient to be able to avoid the melting as mentioned above. However, in the process using the water-cooled copper hearth, the following disadvantage occurs:
In the early stage after being melted, the source material being in contact with the copper wall is cooled too much to be kept at the temperature above the melting point and so formes a deposit on the copper wall. This deposit is increased in the thickness with progress of the deposition process and reaches to a thickness at which the surface of the deposit is scarcely given the water cooling effect through the copper hearth wall. Then the surface of the deposit begins to be melted, so that the thickness thereof decreases gradually. This decrease progresses to an extent at which the melted material being in contact with the deposit surface is cooled sufficiently to be solidified to make the deposit. Thus the such increase and decrease of the deposit occurs cyclically, resulting in a cyclic variation of magnetic properties of the ferromagnetic layer deposited on the substrate.